| dc.description.abstract | In organic light emitting diode, there exists an energy barrier(Schottky energy barrier,φe) between the anode and the organic layer, affecting the charge injection and thus the efficiency and lifetime. By using organic molecules having different size and orientation of the dipole moment to modify the anode surface, one can regulate the interfacial barrier between the electrode and the semiconductor, favoring charge carrier injection and increase efficiency in OLED device.
In this study, organic phosphonic acid molecules(PyOPA, CF3SO2BnPA, CF3BnPA and CF3C3PA) with different sizes of dipole moment were used to prepare self-assembled monolayers on the ITO surface by different methods(vapor phase and solution phase) for subsequent device fabrication. According to previous literatures, the magnitude and direction of the molecular dipole moment of the modifying molecule has an important influence on the work function of ITO electrode. In general, ITO surface treated by oxygen plasma has a work function near 5.8 eV .For ITO modified with a SAM carrying an electron-withdrawing end group(ex trifluoromethyl), the direction of the dipole moment is pointing away from the ITO surface, the work function of the ITO electrode will be increased. The reverse is true for electron-donating group.
However, it was found from my experiment that, further increasing the dipole moment in the molecule does not necessarily increase the work function proportionally. The packing and orientation of the molecules adsorbed on the ITO surface, plays an important role in modulating the work function. By the reflection absorption infrared spectroscopy measurement, it is suggested that the strongly polar molecule adsorbed on the ITO may orient away, from the ITO surface normals, leading to a smaller effective dipole along the surface normal and the work function did not increase as expected so that the device efficiency did not increase as expected.
In gas adsorption experiments, the XPS spectrum suggest that after the same adsorption time, the P 2p and F 1s intensity versus of In 3d3 / 2 is stronger than that for the sample prepared by immersion in the solution, suggesting more than monolayer was obtained. For devices, lower current density and luminance were obtained ,similar result were obtained after sonication of the substrate by ethanol. However, by optical microscope, it was observed that for sample without sonication of the substrate after gas phase adsorption, there are many dark spots in the device. After rinsing the substrate with ethanol and rub gently the surface of substrates by a foam pad, the number of dark spots decreased. Although sonication in ethanol did not completely remove the excessive organic molecules remaining on surface of substrate, quality of the pixel improved. So, it is suggested that solvent rinse is necessary after gas phase adsorption in order to maintain the quality of the device.
| en_US |